Project Summary: Adult heart disease including heart failure and atrial fibrillation are leading causes of morbidity and mortality in the United States and developed world. Although human pluripotent stem cells or other cardiac progenitor cells have great potential to treat these diseases, fully differentiating them into mature functional ventricular or atrial cardiomyocytes for ventricular myocardial regenerative therapies to treat heart failure, for modeling human ventricular and atrial cardiac diseases in a dish, or for therapeutic drug screening has remained a major bottleneck in the biomedical field for realizing these possibilities. Thus, we propose to address this crucial issue through discovering the cellular and molecular mechanisms that mediate the reprogramming of cardiomyocytes into functional ventricular and atrial lineages. Toward this end, we will interrogate the gene regulatory networks that control cardiomyocyte lineage specification during zebrafish cardiac reprogramming and regeneration and then apply this knowledge to identify key cardiac reprogramming factors that guide the differentiation of human pluripotent stem cells into ventricular and atrial cardiomyocytes. Overall, these proposed myocardial reprogramming studies may provide new approaches to address the long-standing issue of how to direct cells into functional ventricular and atrial cardiomyocytes for human cardiac disease modeling and therapeutic screening in cell culture systems as well as for human cardiac regenerative therapies.